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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old November 18th, 2009, 16:03   #1
frugality
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Default A4 thermostat change How-To

There doesn't appear to be a thermostat change how-to, so I took some pics as I changed mine. This was on my A4 Golf, which is probably identical to the Jetta, and probably pretty similar to a Beetle.

Coolant temperature issues:

If you are experiencing coolant temperature variation, it is likely that the coolant temperature sensor is the problem. They are more prone to going bad than are the thermostats. The needle will be low, and will bobble around a bit. I had this happen once, and replacing the coolant temperature sensor fixed the problem. A couple of years later, my coolant temperature was reading low, just within the next hashmark down from 190 degrees, so I thought my new coolant temperature sensor went bad already. I replaced it again and it didn't fix the problem. I replaced the thermostat, and now the gage pegs right to 190 and hold steady there, so it was indeed a bad thermostat this time. When the temperature sensor went bad, it bobbled and varied more. When the thermostat went bad, it was more consistent, staying in a position that was about one hashmark low.


Anyway, without further ado, on to the How-To:

Supplies:
New thermostat. (Part # 044 121 113)
New O-ring. (Part # 038 121 119 B)
New thermostat flange. (Part # 038 121 121)
50/50 mix of G12 (pink) coolant and distilled water.
Catch pan, rags, beer, etc.

Tools:
10mm socket for engine cover nuts.
T-25 and maybe T-30 torx drivers or bits for belly pan.
6mm socket for lower radiator hose clamp at flange.
5mm hex bit (allen bit) on a 6" ratchet extension for flange bolts.
Torque wrench.



* Remove engine cover.

* Jack up the front of your car, or put it on ramps (set your parking brake!), and remove belly pan. (T-25 torx screws, and maybe a T-30 or two...)

* Remove cap from coolant reservoir, to allow air in as the coolant drains.

* Open drain valve at lower left corner of radiator, on the lower radiator hose. It rotates maybe 30-45 degrees counterclockwise, then you pull the whole knob outward to open it up.


This is all the instructions the Bentley manual gives. I only got about 1/2-gallon of coolant out. I expected to get more than that, but in calling r90sKirk, that's about right. A bunch more drained out when I removed the thermostat, and this was impossible to catch, and also got grunge in it from running down the engine, so have some spare 50/50 mix of G12 (pink) coolant and distilled water handy.

I still have my side skirt attached, and it's in the way. I used a funnel to direct the coolant to a catch pan. A piece of rubber hose would do the trick, too, but I didn't have the right size handy.

* Remove clamp from the lower radiator hose at the thermostat flange. This is just underneath the fuel injection pump:


* Remove the 2 bolts holding the thermostat flange to the block, 5mm allen bit on about a 6-inch ratchet extension.

* Remove thermostat flange. The Bentley says to rotate the flange 90 degrees counterclockwise, but this is impossible given the diamond-shaped opening in the bracket that flange passes through. The only thing to do is to pull it straight back, which will damage the flange. (See picture at the end of this how-to.) You should now have this:


* I covered the alternator with aluminum foil to prevent coolant from spilling out on it.

* Remove O-ring. I used a small, curved pick. Coolant will start to leak out, unless someone has any recommendations for draining more coolant out of the block from some location other than the lower radiator drain. Based on how much coolant I had to add later to re-fill it, it seems I dumped another 1/2-gallon, or slightly more. If you only pull back a small bit of the O-ring, the coolant will run out in a more controlled manner, and not gush out. I was only able to catch about half of it in my oil change pan, because it dribbled everywhere down below.

* Remove thermostat. It should now look like this:


* Clean mating surface on the block.

* Install new O-ring and thermostat onto a new coolant flange. To aid installation, the thermostat holds onto 2 pins that are molded into the flange. Looking down the flange:

*** If you are re-using your old flange and have broken the pins/tabs, all is not lost. Refer to this excellent post by 'cattlerepairman':
http://forums.tdiclub.com/showthread.php?t=148452
Orientation of the thermostat in the block is not important. It doesn't have to be lined up with the tabs, they are just there as an assembly aid.


* Install thermostat/O-ring/flange assembly into the engine block.

* Reinstall bolts and torque to 15 Nm (11 ft.-lb.)

* Reattach lower radiator hose to new flange and tighten clamp.

* Double-check that your radiator drain valve is closed.

* Fill with new 50/50 coolant. I did this just through the expansion tank, and was just patient. Some have said that you can hook up a MityVac and draw the air out to help fill the system. Someone elsewhere showed a method of taking the upper radiator hose connection loose near the battery and pouring coolant into both hoses -- into the block and into the radiator. Fill the expansion tank so that it's at the proper level, and reinstall cap.

* Start car and check for leaks. Watch the expansion tank and add coolant if the coolant level drops.

* If everything seems O.K., reinstall belly pan and engine cover. Pat self on back....you have avoided the dealer once again.



Addendum #1:
This is what happens when you pull the old flange off. The original O-ring is pretty well stuck in the block, and holds the thermostat in the block. When you pull the flange, it breaks the 2 thermostat-holder pins.


Addendum #2:
This is a comparison between the OE thermostat ($66 at my local dealer) and an aftermarket thermostat from one of our trusted vendors ($19). The aftermarket thermostat will not attach to the flange. The pointy end is not big enough to catch on the 2 pins. I recommend going with the original VW thermostat. The aftermarket one did come with a new O-ring. The VW one did not, and has to be purchased separately.

The 2 pins of the flange are supposed to engage within the 'arch' on the top of the thermostat. There is no provision for this on the aftermarket thermostat, however. Also, as you can see there are some other differences in construction between the two thermostats.
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Last edited by frugality; November 18th, 2009 at 17:00.
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Old November 21st, 2009, 12:06   #2
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Bump for a great procedure. Instructions were easy to follow. When I was done it took about a 1.25 gallons to get the cooling system filled back up. I know lots of people on here think thermostats don't fail but at 175,000 miles mine was definitely toast. The car wouldn't get up to temp in 50 degree weather. After a quick thermostat change and a test drive it heats right up. Just a heads up for everyone my local dealer had G12 for $24 a gallon. Thats actually a bit cheaper than some of the trusted vendors.
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Old January 4th, 2010, 14:52   #3
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Default Thanks frugality!

Very nice write up frugality.

My car can't get to operating temp even in Houston. Been going on too long and this will help greatly.

Thanks again!
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Old January 4th, 2010, 15:13   #4
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This needs to be moved into the how-to's great write up
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Old February 15th, 2010, 07:33   #5
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I currently have the same issue and the OP did with the needle just barely under the 190 mark. My coolant temp sensor is new so I doubt it is bad. So I guess my thermostat possibly could use a replacement.

Anyway now to a semi related question. Would a bad thermostat effect fuel economy? I'm currently getting 33mpg with a manual transmission and I read on another thread that the thermostat could be an issue if nothing else is.

BTW Great write up.
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Old February 15th, 2010, 07:48   #6
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Excellent write up.

You might try a heavy wire hook through the housing to the thermostat. Never pull on the housing, pull only on the wire hooked to the thermostat, and you will not break the tabs off.
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Old February 15th, 2010, 13:08   #7
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If you have compressed air, you can get nearly a gallon of coolant out.

Remove the return hose from the coolant bottle. Pressurize it slightly with compressed air via blow tip. Coolant will come up into bottle, then drain into radiator. Repeat until very little coolant comes into bottle. Then put cap onto coolant bottle, and pressure the return nipple.

When it's time to refill the sytem, connect a mityvac to the return hose and pull vacuum. Refill bottle as needed. When you get a splash of coolant into the mityvac tubing disconnect it and reconnect return hose to bottle. It will take another bottleful of coolant after car is started to refill system.
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Old February 15th, 2010, 13:39   #8
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I have had good luck with those techniques as well.

If you alternate the air injection between the vent line hose, and the vent line into the coolant reservoir, holding a finger over the other line, you can get even more.

This alternately pressurizes through the vent lines and through the reservoir's bottom line, and moves water around a fair amount.

But still, every time I pull the thermostat or the water pump (whichever I do first), more pee's out...

The only time it has not, is when working on an engine with the head off, I use a vacuum pump to suck the coolant out of the block with a small tube. I do this as I vacuum out the head bolt holes - I get the coolant at the same time.
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Old February 25th, 2010, 09:55   #9
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Great write up indeed. One question I was trying to understand from Bentley. When the thermostat is open, which was does the coolant flow? Out from the thermostat housing, or into the engine through the thermostat?

I have had the same problem for 2 years now. For starters, I had a temp sensor go, which resulted in fluctuations, replaced it, and it fixed it. Then 2 years ago I started noticing that instead of the coolant temp getting up to 190F and staying there, it only gets to about 170F, regardless of whether it is winter or the middle of summer. In fact I can't remember the last time the radiator fans have come on.

The only way I can get the temp to hit 190 is if I let it idle in the driveway for about 10 minutes after I have driven it.

I really hope that a new thermostat will fix this problem.
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Old February 25th, 2010, 10:43   #10
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Turbosprężarka,

At the time you noticed your coolant temp was only 170F, did you also notice a big hit in fuel mileage? Like a 20% or more hit?

My coolant is only at 160F and I'm down to 33 mpg. Used to get 44.

I have a feeling the thermostat is my problem. (Everything else has been eliminated, except injectors/nozzles.)

df
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Old February 25th, 2010, 12:13   #11
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Dieselfuel,
Since Jan 2009 I was getting around 51-52 tank after tank, but it was all highway. I was witnessing low temps with Vag-Com, but at that time they were around 177F during the summer.

Since around September I started noticing my mileage drop off, and I wasn't really sure why. It has gotten much worse since the cold weather started and winter went into full gear. I have gone down to 45-46 on average, and the last time I checked, was when my coolant temp was around 170F.
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Old February 25th, 2010, 14:57   #12
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for a properly operating TDI in 35 weather should the temp needle be at 190 when fully warmed up?

mine dosn't get above 170 ever in lower temps

but it heats up the cabin quickly and maintains 170 well
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Old February 25th, 2010, 16:38   #13
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Yes, it should get to 190.

It will actually read 190 over a temperature range of about 167 to 225 F. This was the way it was set up by VW. The computer actually controls the needle position

So if you are reading 170 on the gauge, then it is actually only about 150 F on VCDS or a scangauge.
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Old February 25th, 2010, 17:15   #14
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Quote:
Originally Posted by Turbosprężarka
Great write up indeed. One question I was trying to understand from Bentley. When the thermostat is open, which was does the coolant flow? Out from the thermostat housing, or into the engine through the thermostat?

coolant flows through the engine, through open t-stat, through radiator, up through the upper hose, back through the engine

but I'm just a newbie
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Old February 25th, 2010, 17:30   #15
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Quote:
Originally Posted by frugality
The 2 pins of the flange are supposed to engage within the 'arch' on the top of the thermostat. There is no provision for this on the aftermarket thermostat, however. Also, as you can see there are some other differences in construction between the two thermostats.
If you broke the tabs why not save the $40-50.
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